1. Field of the Invention
The present invention relates to a thermal head driver system for electrically driving a thermal head, and also relates to an image-forming apparatus having such a thermal head and such a thermal head driver incorporated therein.
2. Description of the Related Art
A thermal head driver system for electrically driving a thermal head is well known. For example, the thermal head is arranged as a line type of thermal head having a plurality of electric resistance elements aligned with each other, and the thermal head driver system is constituted such that the electric resistance elements are selectively and electrically energized in accordance with a single-line of digital image-pixel signals, thereby producing an image on, for example, a thermal sensitive recording sheet.
Usually, the thermal head driver system includes a shift register, and a latch circuit connected in parallel to the shift register. The single-line of digital image-pixel signals is serially inputted to and is temporarily stored in the shift register, and the stored digital image-pixel signals are then shifted to the latch circuit. The shifted digital image-pixel signals are latched by the latch circuit, and are stably held therein. The latch circuit is provided with a plurality of output terminals corresponding to a number of the digital image-pixel signals held therein, and each of the output terminals outputs a high-level signal only when a corresponding digital image-pixel signal has a value xe2x80x9c1xe2x80x9d.
The thermal head driver system also includes a plurality of AND-gate circuits each having two input terminals and an output terminal, and a plurality of switching circuits associated with the AND-gate circuits, respectively. One of the input terminals of each AND-gate circuit is connected to a corresponding one of the output terminals of the latch circuit, and the other input terminal of each AND-gate circuit is wired so as to receive a strobe signal having a predetermined pulse width. The output terminal of each AND-gate circuit is connected to the switching circuit associated therewith. Each of the electric resistance elements of the line thermal head is connected to an electric power source through a corresponding switching circuit.
With this arrangement of the thermal head driver system, when one of the digital image-pixel signals held in the latch circuit has a value xe2x80x9c1xe2x80x9d, so that a high-level signal is outputted from a corresponding output terminal of the latch circuit, a corresponding AND-gate circuit is opened so that a corresponding switching circuit is turned ON, whereby a corresponding electric resistance element is electrically energized over a period corresponding to the pulse width of the strobe signal so as to be heated to a predetermined temperature. On the other hand, when one of the digital image-pixel signals held in the latch circuit has a value xe2x80x9c0xe2x80x9d, a corresponding AND-gate circuit is maintained at a closed state, so that a corresponding switching circuit also maintains an OFF state, whereby a corresponding electric resistance element cannot be electrically energized.
Conventionally, one thermal head necessarily involves one thermal head driver system as mentioned above, and these two elements are inseparably related to each other. In other words, a thermal head driver system is provided for the purpose of driving only a single thermal head.
An object of the present invention is to provide a novel thermal head driver system arranged to selectively drive at least two thermal heads in accordance with at least two types of image information data, respectively, without a thermal head driver system being necessary for each thermal head.
Another object of the present invention is to provide a thermal image-forming apparatus including at least two thermal heads, which are selectively driven by the above-mentioned novel thermal head driver system in accordance with at least two types of image information data.
In accordance with an aspect of the present invention, there is provided a thermal head driver system that cyclically and independently drives each of at least two thermal heads having each a plurality of electric resistance elements. The thermal head driver system comprises a storage system that cyclically stores an image information data, the image information data cyclically being each of at least two types of image information data, respectively corresponding to the at least two thermal heads, and a selector system that cyclically and correspondingly selects which thermal head should be driven in accordance with the cyclical storage of the at least two types of image information data in the storage system, such that the electric resistance elements of the thermal head, selected by the selector system, are selectively and electrically energized in accordance with a corresponding type of image information data cyclically stored in the storage system. Preferably, the thermal head driver system further comprises a determiner system that determines a time period over which the thermal head, selected by the selector system, is driven.
The selector system may comprise a signal generator that generates at least two selection-control signals, each of which changes between a first level and a second level, and the cyclical selection of the driving of the at least two thermal heads is performed in accordance with a combination of the levels of the at least two selection-control signals. In this case, when at least one of the at least two selection-control signals is changed from the first level to the second level, one of the thermal heads is correspondingly selected to be driven by the selector system. Also, when the at least two selection-control signals are kept at the first level, none of the thermal heads are selected to be driven by the selector system.
In accordance with another aspect of the present invention, there is provided an image-forming apparatus that forms an image on an image-forming substrate that includes a base member and a layer of microcapsules, coated over the base member, containing a first type of microcapsule filled with a first monochromatic dye, and a second type of microcapsule filled with a second monochromatic dye, the first type of microcapsule exhibiting a first pressure/temperature characteristic such that, when the first type of microcapsule is squashed under a first pressure at a first temperature, the first type of microcapsule breaks discharging the first dye, the second type of microcapsule exhibiting a second pressure/temperature characteristic such that, when the second type of microcapsule is squashed under a second pressure at a second temperature, the second type of microcapsule breaks discharging the second dye. The image-forming apparatus comprises a first pressure applicator that locally exerts the first pressure on the layer of microcapsules, a second pressure applicator that locally exerts the second pressure on the layer of microcapsules, a first thermal head that is driven such that a first localized area of the layer of microcapsules, on which the first pressure is exerted by the first pressure applicator, is heated to the first temperature in accordance with a first image-information data, such that the first type of microcapsule in the first localized area is selectively broken, a second thermal head that is driven such that a second localized area of the layer of microcapsules, on which the second pressure is exerted by the second pressure applicator, is heated to the second temperature in accordance with a second image-information data, such that the second type of microcapsule in the second localized area is selectively squashed, and a thermal head driver system that cyclically and independently controls the driving of the first and second thermal heads, and that is arranged in accordance with the first-mentioned aspect of the present invention.
The layer of microcapsules may further contains a third type of microcapsule filled with a third monochromatic dye, the third type of microcapsule exhibiting a third pressure/temperature characteristic such that, when the third type of microcapsule is squashed under a third pressure at a third temperature, the third type of microcapsule breaks discharging the third dye. In this case, the image-forming apparatus comprises further comprises a third pressure applicator that locally exerts the third pressure on the layer of microcapsules, and a third thermal head that is driven such that a third localized area of the layer of microcapsules, on which the third pressure is exerted by the third pressure applicator, is heated to the third temperature in accordance with a third image-information data, such that the third type of microcapsule in the third localized area is selectively squashed. Also, the thermal head driver system cyclically and independently controls the driving of the first, second and third thermal heads, the storage system cyclically stores an image information data, the image information data cyclically being each of the first, second and third image information data, and the selector system cyclically and correspondingly selects which thermal head should be driven in accordance with the cyclical storage of the first, second and third image information data in the storage system.
The selector system may comprise a signal generator that generates two selection-control signals, each of which changes between a first level and a second level, and the cyclical selection of the driving of the first, second and third thermal heads is performed in accordance with a combination of the levels of the selection-control signals. When at least one of the two selection-control signals is changed from the first level to the second level, one of the first, second and third thermal heads is correspondingly selected to be driven by the selector system. Also, when all of the two selection-control signals are kept at the first level, none of the first, second and third thermal heads is selected to be driven by the selector system.